Many natural gas formations contain hydrogen sulfide, methane, carbon dioxide and other impurities or contaminants. Typically, these impurities or contaminants must be removed from the natural gas before the natural gas is suitable for use. Similarly, process gases, such as those resulting from gas refining processes or as a by-product of other industrial processes, may contain hydrogen sulfide, methane, carbon dioxide and other impurities or contaminants. These impurities or contaminants must also be removed or otherwise addressed prior to the disposal or further processing and handling of the process gas.
In this context, the term “sour gas” as used herein refers to a natural gas which contains quantities of methane and hydrogen sulfide, as well as one or more impurities or contaminants such as carbon dioxide. Further, the term “acid gas” as used herein refers to process gas or sour gas from which substantially all of the methane has been extracted, leaving hydrogen sulfide and carbon dioxide as the principal components. Methane is typically extracted from sour gas using conventional amine extraction processes known to those skilled in this area. The removal of the hydrogen sulfide from the sour gas, referred to as gas sweetening, as well as the production of acid gas by industry, results in quantities of hydrogen sulfide containing gas which must be disposed of.
In smaller plants or refineries, the hydrogen sulfide is often flared to the atmosphere. However, flaring of the hydrogen sulfide converts it to sulfur dioxide which is a major component of acid rain. Thus, flaring of the hydrogen sulfide, particularly in large quantities, is not desirable. Accordingly, in larger plants and refineries, the hydrogen sulfide is further processed in various sulfur extraction and recovery systems in order to reduce the volume of hydrogen sulfide to be disposed of. However, these systems tend to be costly and still tend to result in significant quantities of hydrogen sulfide which are required to be flared.
For instance, a conventional sulfur recovery process known as the “Claus Process” utilizes a two step process which results in the production of elemental sulfur and a “tail gas” from the hydrogen sulfide. The first step is a thermal step comprised of combining the gas, typically acid gas, with oxygen and heating it to the necessary temperature for combustion. The combustion products then undergo the second catalytic step. Specifically, the catalytic step is comprised of combining the combustion products with a catalyst which results in the production of elemental sulfur and a tail gas which includes a residual quantity of hydrogen sulfide. The tail gas is typically incinerated in a flare stack or a furnace and vented to the atmosphere.
Thus, it is a goal of the Claus Process to minimize the amount of hydrogen sulfide which is contained in the tail gas and thereby minimize the production of sulfur dioxide during flaring. Accordingly, specialized or improved catalysts have been developed for the second step which achieve higher conversion rates to sulfur. In addition, where desired, a third step, being a tail gas recovery step, may be performed on the tail gas following the catalytic step to further reduce the amount of residual hydrogen sulfur in the tail gas. For instance, conventional amine extraction processes or conventional amine plants or regenerators may be used to selectively remove further amounts of any residual hydrogen sulfide from the tail gas. In addition, a “tail gas recovery process” may be required to convert sulfur-containing impurities, such as COS, CS2, SO2 and sulfur aerosol, back into hydrogen sulfide for recycling back to the Claus Process.
However, the use of specialized catalysts and further processing steps tends to increase the overall cost of the sulfur recovery process, as well as increase the overall energy requirements of the process. Further, some sulfur dioxide continues to be produced and released to the atmosphere as a result of the flaring of unconverted hydrogen sulfide.
Further alternative approaches have also been utilized. However, as with the Claus Process, none of these alternative approaches have been found to be fully satisfactory.
For example, sour gas, acid gas and other industrial waste gases have been fed to a reactor, such as a plasma reactor or thermoelectric reactor, which causes the dissociation of the hydrogen sulfide into hydrogen and sulfur. Thus, both valuable sulfur products and potentially valuable hydrogen products are produced. However, the dissociation process also produces a number of other impurities or contaminants in the product stream resulting from the sour gas including COS, CS2, SO2, CO, CO2, H2O and sulfur aerosols. Examples of the use of the dissociation process include European Patent Application 1,085,075 A1 published Mar. 21, 2001 by ABB Research Ltd, U.S. Pat. No. 5,843,395 issued Dec. 1, 1998 to Wang and PCT International Publication WO 00/56441 published Sep. 28, 2000 by Agarwal et. al.
U.S. Pat. No. 5,211,923 issued May 18, 1993 to Harkness et. al. particularly relates to a process which utilizes a microwave plasma reactor to produce sulfur and hydrogen from sour gas. The sour gas entering the plasma reactor includes a variety of impurities or contaminants including methane, carbon dioxide, hydrogen sulfide and various other substances containing carbon, hydrogen, sulfur and oxygen. As a result, the plasma reactor may utilize significant energy dissociating a number of compounds in the sour gas, in addition to the hydrogen sulfide. Further, the dissociation may well result in the production of various undesirable impurities or contaminants in the product stream as noted above.
Accordingly, Harkness et. al. specifically incorporates a catalytic reduction unit following the plasma reactor which serves to convert sulfur-containing impurities which exit the plasma reactor into a “hydrogen sulfide enhanced stream.” Hydrogen is then separated from the hydrogen sulfide enhanced stream in a “purification step” and the remaining hydrogen sulfide is recycled back to the plasma reactor.
Finally, as indicated above, in addition to the production of sulfur products, hydrogen sulfide has been recognized as a potential feed stock for the production of hydrogen products. The development of hydrogen sulfide processing technologies for the production of hydrogen is also desirable given the increasing importance of, and need for, hydrogen for chemical processing and as a potential fuel source.
As a result, there is a need in the industry for a process and an apparatus for obtaining a hydrogen product, comprised of elemental hydrogen, from a feed gas comprised of hydrogen sulfide. Further, there is a need for a process and the apparatus which also obtain a sulfur product comprised of elemental sulfur. As well, there is a need for such a process and apparatus to be relatively energy and cost efficient and which minimize or reduce the amount of undesirable gases required to be flared or vented to the atmosphere.